There has been known a ball joint which includes a ball stud having a spherical head portion and a shaft portion, and a socket coupled with the spherical head portion of the ball stud via a ball seat, and in which the ball stud can turn in relation to the socket about the spherical center of the spherical head portion. Such a ball joint is shown in, for example, Japanese Patent Application Laid-Open (kokai) No. H11-182530.
In the ball joint disclosed in the publication, the ball seat is secured to the socket, and the spherical head portion of the ball stud is slidably supported by a spherical surface formed on the ball seat. In this ball joint, if frictional engagement force produced between the spherical head portion of the ball stud and the ball seat is set high, when the ball stud rotates about its center axis, its rotational angle (deg) and rotational torque (Nm) have a relation as indicated by characteristic line a in FIG. 4. If the frictional engagement force between the spherical head portion of the ball stud and the ball seat is set low, the relation changes as indicated by characteristic line b in FIG. 4.
Incidentally, the frictional engagement force between the spherical head portion of the ball stud and the ball seat can be changed by varying the material (friction coefficient) of the ball seat, the engagement force (surface pressure) between the spherical head portion of the ball stud and the ball seat, grease interposed between the spherical head portion of the ball stud and the ball seat, and the like. However, in any case, the ball stud does not rotate in relation to the ball seat and the socket until the rotational torque reaches a set value (e.g., Ta, Tb in FIG. 4). Therefore, in the case where such a ball joint is applied to a portion which constitutes a kingpin of a steerable wheel in a suspension system for a vehicle, satisfactory steering feeling may fail to be attained, because the ball stud does not rotate until the rotational torque reaches a set value (e.g., Ta, Tb in FIG. 4).